Telecommunications products often make use of radio frequency (RF) signals in the megahertz frequency range to achieve cordless portability. Electronic circuits to permit the transmission and reception of such signals are known as RF circuits. Such circuits generate electro-magnetic interference (EMI) in the RF frequency range while in operation. The EMI generated by an RF circuit may interfere with the performance of other proximate RF or non-RF circuitry.
The degree of interference is related to the sensitivity of the proximate circuit to EMI, the operating frequency of the interfering RF circuit, the level of EMI generated by the interfering circuit and the proximity of the two circuits.
In cellular radios, both high frequency and high dynamic range is required. Receive signal power can be less than -100 dBm and oscillator and transmit signals may vary from 0 to 27 dBm. At such low power levels, circuits are very sensitive to EMI.
Generally, telecommunications products using RF frequency transmissions are constrained by regulatory requirements to operate along certain frequency bands. Furthermore, because of the requirement of portability, the size of such telecommunications products is kept as small as possible. This requires that the various circuits in such products be kept in close proximity, often on the same printed circuit board (PCB).
Accordingly, the potential that the performance of other circuits in the product will be impaired due to interference can be meaningfully reduced only by limiting the level of EMI to which these proximate circuits are exposed. Moreover, many jurisdictions now impose stringent regulatory maximum thresholds on the amount of EMI that an electronic device may radiate. Therefore, considerable efforts are expended to develop effective mechanisms to shield the RF circuitry in such products.
The concept of EMI shielding is based on the principle that a source of electro-magnetic energy may be contained by an electrically conductive enclosure. The completeness of the enclosure will govern the effectiveness of the containment of the EMI radiated by the source.
Ideally, therefore, an operating RF circuit may be completely shielded by completely enclosing the circuit in an electrically conductive case, such as a sealed metal box. However, as a practical matter, such a shielded circuit would be of little use as the RF transmissions which the circuit was designed to either transmit or receive would also be contained or excluded by the shield.
Thus, at a minimum, the desired RF signal to be transmitted or received must be permitted to escape or enter the EMI shield as the case may be. Moreover, an RF circuit typically requires power and control signal inputs, and/or generates control signal outputs. Therefore the EMI shield typically must permit a plurality of electrical connections across the shield boundary.
Furthermore, as indicated previously, size constraints on occasion dictate that the other circuitry in the telecommunications device co-exist on the same PCB, yet remain isolated from the radiated EMI of the RF circuitry. Therefore the EMI shield cannot as a practical matter be a solid conductive enclosure about the RF circuit to be shielded.
Previous approaches to effecting an EMI shield involved the combination of a metal conductive enclosure to surround a printed circuit board and the use of a plurality of connectors to pass signals beyond the boundaries of the enclosure.
To accommodate the co-existence of a plurality of circuits on the same PCB, the circuits were separated by a metallized boundary on both faces of the PCB, and the use of integral metal walls on both pieces of the enclosure which were vertically co-planar with the metal boundary strips when the enclosure was placed about the PCB. Periodically spaced vias were drilled through the metal boundaries and the PCB. Thus, when the enclosure was applied about the PCB, the gasket lining the walls on one of the enclosure components extended slightly through the vias into the PCB itself and into electrical contact with a gasket along the corresponding wall of the other piece of the enclosure. The effect of this enclosure system was to provide an effectively continuous conductive enclosure about each of the circuits separated by the metallized boundaries.
A plurality of shielded connectors was used in conjunction with this enclosure system as described above, to permit power and control signals to pass between circuits on the same PCB, from a circuit within a PCB to a circuit off the PCB.
Typically, passage of such signals has been effected by the use of a shielded connector comprising a suitable electrical connector bearing the signal to be passed across the zone boundary, and low pass filtering means at the zone boundary, whereby a capacitance is established from the conductor to the shield enclosure which is usually connected to electrical ground, and an impedance element in series between the conductor and the PCB where the signal to be passed appeared.
The impedance element would be either a resistor or an inductor or a combination of a resistor and inductor depending upon the nature of the signal to be passed, with the aim of minimizing the voltage drop across the impedance element. Thus, where the signal has a large DC current component one would prefer to use an inductor, while a resistor would be preferable for a control signal that does not require high current.
The interposition of a capacitor between the conductor and the electrically grounded enclosure provided a low impedance path at high frequencies so that any unwanted RF EMI will take the path of least resistance through the capacitor, and not radiate through any gap in the shield enclosure required to pass the connector across the zone boundary. The equivalent electrical circuit is shown in FIG. 1a when a resistor is used for the impedance element and in FIG. 1b when an inductor is used for the impedance element.
Initial implementations of such shielded connectors involved the passage of suitable electrical connector through a hole drilled in the metal wall, and the connection of discrete capacitors and resistors at each end of the connector.
Later approaches included the development of integral filter connectors comprising a typically cylindrical metal sheath filled with a high dielectric material through which a conductor is passed axially. Thus, capacitance is created radially outward from the filter conductor to the metal sheath. Any required resistance element was obtained by constructing the conductor out of material of suitable resistivity. An inductive element was implemented by forming the filter conductor in a helical coil about the axis of the metal sheath.
When the connector was inserted into a hole drilled through the wall, a metal sheath remained in relatively continuous conductive contact with the wall through which it was passed. Thus the capacitance extending radially outward from the conductor to the sheath effectively extended to electrical ground through the RFI enclosure. The exposed end of the filter conductor were directly soldered to the conductive traces at which the signal to be passed appeared.
While such implementations were effective, they were impractical from a manufacturing point of view, because the process was labour intensive and not susceptible to automation. Moreover, such implementations required significant amounts of space, which is, as discussed above, typically at a premium. Furthermore, because such implementations were not planar, the top piece of the enclosure could not conveniently be removed and replaced as required, because of the connections between the filter connector and the conductive traces. Accordingly, an alternative planar method of implementing a shielded connector more conducive to commercial manufacturing methods was required.
One attempt to implement a planar shielded connector involved the modification of the PCB to create a conductive buried layer interior to the PCB and parallel to its planar surface. The buried layer would typically pass under a zone boundary and its layout would be dependent upon the location of the signal to be passed. The buried layer would not intersect with any of the vias drilled through the printed circuit board, nor with the holes through which the leads of the various integrated circuit components would extend.
To access this buried layer, additional connector vias were drilled partially through the PCB and terminating in the buried layer. There would be typically two connector vias one on each side of the zone boundary proximate to the conductive trace from or to which the signal to be passed appeared. A discrete impedance element was soldered between the conductive trace and the conductor via and a discrete capacitor was soldered between the connector via and the via at the zone boundary.
The interposition of the capacitor between the connector via and the boundary via and of an impedance element between the connector via and the signal trace created on either side of the boundary, a low pass filter equivalent to one of the circuits shown in FIGS. 1a and 1b.
This approach, however, suffers from a number of disadvantages. First, it requires an additional and expensive doping step in the manufacture of the PCB to permit the deposition of the buried layer. Second, design rules relating to such structures typically require, for reasons of accurate registration, that the length of the capacitor lead soldered to the boundary via to exceed a certain minimum distance. The physical distance between the capacitor and the connector the boundary via deleteriously impacts the effectiveness of the filtering obtained using this approach in two respects.
First, with high frequency EMI components, the wavelength will correspondingly be very small. Accordingly, the mechanical distance of the capacitor lead may represent a significant fraction or number of wavelengths of the EMI, and a parasitic inductance along the capacitor lead would be introduced.
This inductance varies in proportion to both the frequency of the EMI component and the length of the capacitor lead. As the parasitic inductance is increased, the capacitance needs to correspondingly be reduced in order to achieve the desired effective frequency of the filter.
However, as the capacitance is decreased, the effective filter bandwidth is similarly decreased, as is the degree of attenuation of the EMI sought to be suppressed. Thus, in order to obtain in excess of 20 dB of attenuation, series resonant components need to be chosen.
Second, the length of the conductor between the capacitor and the EMI shield enclosure (including the top boundary strip 12) constitutes a conductor which is not shielded by the EMI shield enclosure, as it lies physically beyond the low pass filter structure. As this distance is increased, the possibility of EMI emanating along this conductor is also increased, with a corresponding degradation in shielding performance.